Fungi, and especially filamentous fungi, are widely used commercially as host cells for the expression of proteins which are secreted extracellularly. In particular, species belonging to the genus Aspergillus have a long history of commercial use for the production of endogenous and lately also heterologous proteins.
One disadvantage frequently encountered with microorganisms used as host cells is the inherent production and secretion of high levels of proteolytic enzymes which may result in reduced yields of a protein product of interest due to proteolysis.
Various solutions to circumvent this have been envisaged. For example, one could delete or disrupt the genes encoding the various endogenous proteases. WO 90/00192 (Genencor Inc.) describes mutant filamentous fungal hosts which have been rendered incapable of secreting enzymatically active aspartic protease. By such mutation, it was shown that the yield of the heterologous polypeptide, bovine chymosin, was increased. EP 574 347 (Ciba Geigy AG) describes an Aspergillus host defective in a serine protease of the subtilisin type.
However, it is well known that fungi produce a large number of proteases in addition to the two herein mentioned. Consequently, strains of filamentous fungi exhibiting no or very low levels of proteolytic activity originating from other proteases are still needed.